Isolated converter with high boost ratio

ABSTRACT

An isolated converter with high boost ration includes a transformer, a first bridge arm, a second bridge arm, and a boost circuit. The transformer includes a secondary side having a secondary side first node and a secondary side second node. The first bridge arm includes a first diode and a second diode. The second bridge arm includes a third diode and a fourth diode. The boost circuit includes at least one fifth diode coupled between the first bridge arm and the secondary side second node, at least one sixth diode coupled between the second bridge arm and the secondary side first node, and at least two capacitors coupled to the secondary side first node and the secondary side second node.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional PatentApplication No. 62/860,650, filed Jun. 12, 2019, and claims priority toCN202010248753.X filed Apr. 1, 2020. The entire disclosures of the aboveapplications are all incorporated herein by reference.

BACKGROUND Technical Field

The present disclosure relates to an isolated converter, and moreparticularly to an isolated converter with high boost ratio.

Description of Related Art

The statements in this section merely provide background informationrelated to the present disclosure and do not necessarily constituteprior art.

Please refer to FIG. 1, which shows a circuit block diagram of aconventional non-isolated boost converter. If a high voltage conversionratio (boost ratio) is required, for example more than 10 times, two ormore than two boost (DC-to-DC) converters are used to implement therequirement of high boost ratio. As shown in FIG. 1, a first DC boostconverter and a second DC boost converter are used. However,disadvantages of the multi-stage boost converters are higher circuitcomponent costs and lower conversion efficiency.

Please refer to FIG. 2, which shows a circuit diagram of a conventionalpush-pull converter. The push-pull converter has an advantage ofelectrical isolation by an isolated transformer. By adjusting the numberof coil turns of the isolated transformer, the high boost ratio (voltageconversion ratio) of the push-pull converter can be implemented. If thevoltage conversion ratio is too high, however, excessive number of thesecondary-side coil turns causes an increase in the leakage inductanceof the isolated transformer, resulting in higher stress on the switchand/or the diode, and therefore the components with higher rated voltageneed to be used. Further, the turned-on resistance of the switch becomeslarger cause increased circuit costs, reduced efficiency, and largertransformer volume.

Please refer to FIG. 3, which shows a circuit diagram of a conventionalfull-bridge LLC resonant converter. The switches of the LLC resonantconverter can operate under ZVS (zero voltage switching) condition, andusually the switching frequency is designed near the resonance point. Byadjusting the number of coil turns of the isolated transformer, the highvoltage conversion ratio of the LLC resonant converter can beimplemented. If the voltage conversion ratio is too high, however,excessive number of the coil turns causes the increased coil turns ofthe transformer, increased volume of the transformer, and reducedefficiency. Moreover, the full-bridge LLC resonant converter shown inFIG. 3 provides the first- and third-quadrant operations of thetransformers. The quadrant here refers to the main operation area of thetransformer in the B-H curve (hysteresis phenomenon is ignored).

Please refer to FIG. 4, which shows a circuit diagram of a conventionalfull-bridge phase-shift converter. The switches of the full-bridgeconverter can operate under ZVS condition by phase control techniques.In comparison with the push-pull converter, the full-bridge converterhas fewer switching losses. By adjusting the number of coil turns of theisolated transformer, the required output voltage of the full-bridgeconverter can be implemented. In order to implement the high boost ratio(voltage conversion ratio), however, the excessive number of the coilturns causes larger transformer volume, higher stress of thesemiconductor switch, and lower conversion efficiency.

In the above several common isolated converters, the isolatedtransformer thereof operates in the first quadrant and the thirdquadrant. The present disclosure proposes an isolated converter withhigh boost ratio that can operate in the first quadrant and the thirdquadrant. The high boost ratio of the isolated converter can beimplemented by charging capacitors to establish a high output voltage,thereby effectively reducing the number of coil turns of the isolatedtransformer. Accordingly, it is to increase efficiency of the boostcircuit, reduce circuit component costs, and reduce occupied volume dueto the reduction of the number of coil turns of the isolatedtransformer.

SUMMARY

An object of the present disclosure is to provide an isolated converterwith high boost ratio to solve the above-mentioned problems.

In order to achieve the above-mentioned object, the isolated converterwith high boost ratio includes a transformer, a first bridge arm, asecond bridge arm, and a boost circuit. The transformer includes asecondary side having a secondary side first node and a secondary sidesecond node. The first bridge arm has a first diode and a second diode.A cathode of the first diode is coupled to a DC positive output node, ananode of the first diode is coupled to a first bridge arm upper node. Ananode of the second diode is coupled to a DC negative output node, and acathode of the second diode is coupled to a first bridge arm lower node.The second bridge arm has a third diode and a fourth diode. A cathode ofthe third diode is coupled to the DC positive output node, an anode ofthe third diode is coupled to a second bridge arm upper node. An anodeof the fourth diode is coupled to the DC negative output node, and acathode of the fourth diode is coupled to a second bridge arm lowernode. The boost circuit includes at least one fifth diode, at least onesixth diode, and at least two capacitors. The at least one fifth diodeis coupled between the first bridge arm and the secondary side secondnode. The at least one sixth diode is coupled between the second bridgearm and the secondary side first node. The at least two capacitors arecoupled to the secondary side first node and the secondary side secondnode.

Accordingly, the isolated converter with high boost ratio is provided toincrease efficiency of the boost circuit, reduce circuit componentcosts, and reduce occupied volume due to the reduction of the number ofcoil turns of the isolated transformer.

Another object of the present disclosure is to provide an isolatedconverter with high boost ratio to solve the above-mentioned problems.

In order to achieve the above-mentioned object, the isolated converterwith high boost ratio includes a transformer, a first bridge arm, asecond bridge arm, and a boost circuit. The transformer includes asecondary side having a secondary side first node and a secondary sidesecond node. The first bridge arm has a first switch and a secondswitch. A first end of the first switch is coupled to a DC positiveoutput node, a second end of the first switch is coupled to a firstbridge arm upper node, a first end of the second switch is coupled to aDC negative output node, and a second end of the second switch iscoupled to a first bridge arm lower node. The second bridge arm has athird switch and a fourth switch. A first end of the third switch iscoupled to the DC positive output node, a second end of the third switchis coupled to a second bridge arm upper node, a first end of the fourthswitch is coupled to the DC negative output node, and a second end ofthe fourth switch is coupled to a second bridge arm lower node. Theboost circuit includes at least one fifth switch, at least one sixthswitch, and at least two capacitors. The at least one fifth switch iscoupled between the first bridge arm and the secondary side second node.The at least one sixth switch is coupled between the second bridge armand the secondary side first node. The at least two capacitors arecoupled to the secondary side first node and the secondary side secondnode.

Accordingly, the isolated converter with high boost ratio is provided toincrease efficiency of the boost circuit, reduce circuit componentcosts, and reduce occupied volume due to the reduction of the number ofcoil turns of the isolated transformer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the present disclosure as claimed. Otheradvantages and features of the present disclosure will be apparent fromthe following description, drawings and claims.

BRIEF DESCRIPTION OF DRAWINGS

The present disclosure can be more fully understood by reading thefollowing detailed description of the embodiment, with reference made tothe accompanying drawing as follows:

FIG. 1 is a circuit block diagram of a conventional non-isolated boostconverter.

FIG. 2 is a circuit diagram of a conventional push-pull converter.

FIG. 3 is a circuit diagram of a conventional full-bridge LLC resonantconverter.

FIG. 4 is a circuit diagram of a conventional full-bridge phase-shiftconverter.

FIG. 5 is a single-bus circuit structure of an isolated converter withhigh boost ratio according to a first embodiment of the presentdisclosure.

FIG. 6 is a single-bus circuit structure of the isolated converter withhigh boost ratio according to a second embodiment of the presentdisclosure.

FIG. 7 is a schematic diagram of current paths of a first-quadrantoperation in FIG. 5.

FIG. 8 is a schematic diagram of current paths of a third-quadrantoperation in FIG. 5.

FIG. 9 is a single-bus circuit structure of the isolated converter withhigh boost ratio according to a third embodiment of the presentdisclosure.

FIG. 10 is a single-bus circuit structure of the isolated converter withhigh boost ratio according to a fourth embodiment of the presentdisclosure.

FIG. 11 is a dual-bus circuit structure of the isolated converter withhigh boost ratio according to the present disclosure.

FIG. 12 is a circuit diagram of a push-pull converter having thesingle-bus circuit structure in FIG. 5.

FIG. 13 is a circuit diagram of a full-bridge converter having thesingle-bus circuit structure in FIG. 5.

FIG. 14 is a circuit diagram of a full-bridge LLC resonant converterhaving the single-bus circuit structure in FIG. 5.

FIG. 15 is a circuit diagram of a push-pull converter having thedual-bus circuit structure in FIG. 11.

FIG. 16 is a circuit diagram of a full-bridge converter having thedual-bus circuit structure in FIG. 11.

FIG. 17 is a circuit diagram of a full-bridge LLC resonant converterhaving the dual-bus circuit structure in FIG. 11.

DETAILED DESCRIPTION

Reference will now be made to the drawing figures to describe thepresent disclosure in detail. It will be understood that the drawingfigures and exemplified embodiments of present disclosure are notlimited to the details thereof.

Please refer to FIG. 5, which shows a single-bus circuit structure of anisolated converter with high boost ratio according to a first embodimentof the present disclosure. The isolated converter includes a transformerT1, a first bridge arm Lb1, a second bridge arm Lb2, and a boost circuit10. The transformer T1 includes a secondary side having a secondary sidefirst node Ps1 and a secondary side second node Ps2. The first bridgearm Lb1 includes a first diode D1 and a second diode D2. A cathode ofthe first diode D1 is coupled to a DC positive output node +Vb, an anodeof the first diode D1 is coupled to a first bridge arm upper node Pu1,an anode of the second diode D2 is coupled to a DC negative output node−Vb, and a cathode of the second diode D2 is coupled to a first bridgearm lower node Pw1. The second bridge arm Lb2 includes a third diode D3and a fourth diode D4. A cathode of the third diode D3 is coupled to theDC positive output node +Vb, an anode of the third diode D3 is coupledto a second bridge arm upper node Pu2, an anode of the fourth diode D4is coupled to the DC negative output node −Vb, and a cathode of thefourth diode D4 is coupled to a second bridge arm lower node Pw2.

The boost circuit 10 includes at least one fifth diode D5, at least onesixth diode D6, and at least two capacitors. The at least one fifthdiode D5 is coupled between the first bridge arm Lb1 and the secondaryside second node Ps2. The at least one sixth diode D6 is coupled betweenthe second bridge arm Lb2 and the secondary side first node Ps1. The atleast two capacitors are coupled to the secondary side first node Ps1and the secondary side second node Ps2.

As shown in FIG. 5, the number of the at least two capacitors is two,including a first capacitor C1 and a second capacitor C2. Two ends ofthe first capacitor C1 are coupled to the secondary side first node Ps1and the first bridge arm upper node Pu1, and two ends of the secondcapacitor C2 are coupled to the secondary side second node Ps2 and thesecond bridge arm upper node Pu2. A cathode of the fifth diode D5 iscoupled to the first bridge arm upper node Pu1, and an anode of thefifth diode D5 is coupled to the secondary side second node Ps2; acathode of the sixth diode D6 is coupled to the second bridge arm uppernode Pu2, and an anode of the sixth diode D6 is coupled to the secondaryside first node Ps1.

As shown in FIG. 6, the number of the at least two capacitors is two,including a first capacitor C1 and a second capacitor C2. Two ends ofthe first capacitor C1 are coupled to the secondary side first node Ps1and the first bridge arm lower node Pw1, and two ends of the secondcapacitor C2 are coupled to the secondary side second node Ps2 and thesecond bridge arm lower node Pw2. An anode of the fifth diode D5 iscoupled to the first bridge arm lower node Pw1, and a cathode of thefifth diode D5 is coupled to the secondary side second node Ps2; ananode of the sixth diode D6 is coupled to the second bridge arm lowernode Pw2, and a cathode of the sixth diode D6 is coupled to thesecondary side first node Ps1.

The diode-capacitor circuit, which is coupled to the secondary side ofthe transformer T1, is proposed to charge the capacitors through thesecondary side coil turns, thereby reducing coil turns, increasingconversion efficiency, and reducing occupied volume. Further, thediode-capacitor circuit can be used in a variety of isolated converterscapable of operating both in a first quadrant and a third quadrant.

Please refer to FIG. 7, which shows a schematic diagram of current pathsof a first-quadrant operation in FIG. 5. When the isolated converteroperates in a first quadrant, a main current Ic1 flows through thesecondary side of the transformer T1, the second capacitor C2, the thirddiode D3, the DC positive output node +Vb, the DC negative output node−Vb, the second diode D2, and the secondary side. A pre-charge currentIc2 flows through the secondary side of the transformer T1, the fifthdiode D5, the first capacitor C1, and the secondary side. In particular,the first capacitor C1 is charged by the pre-charge current Ic2.

In addition, when the isolated converter shown in FIG. 6 operates in afirst quadrant (not shown), a main current flows through the secondaryside of the transformer T1, the third diode D3, the DC positive outputnode +Vb, the DC negative output node −Vb, the second diode D2, thefirst capacitor C1, and the secondary side. A pre-charge current flowsthrough the secondary side of the transformer T1, the second capacitorC2, the sixth diode D6, and the secondary side.

Please refer to FIG. 8, which shows a schematic diagram of current pathsof a third-quadrant operation in FIG. 5. When the isolated converteroperates in a third quadrant, a main current Ic1 flows through thesecondary side of the transformer T1, the first capacitor C1, the firstdiode D1, the DC positive output node +Vb, the DC negative output node−Vb, the fourth diode D4, and the secondary side. A pre-charge currentIc2 flows through the secondary side of the transformer T1, the sixthdiode D6, the second capacitor C2, and the secondary side. Similarly,when the isolated converter shown in FIG. 6 operates in a third quadrant(not shown), a main current flows through the secondary side of thetransformer T1, the first diode D1, the DC positive output node +Vb, theDC negative output node −Vb, the fourth diode D4, the second capacitorC2, and the secondary side. A pre-charge current flows through thesecondary side of the transformer T1, the first capacitor C1, the fifthdiode D5, and the secondary side.

Please refer to FIG. 9, which shows a single-bus circuit structure ofthe isolated converter with high boost ratio according to a thirdembodiment of the present disclosure, and the circuit structure of FIG.9 is a combination of the circuit structure of FIG. 5 and the circuitstructure of FIG. 6. As shown in FIG. 9, the number of the at least twocapacitors is four, including a first capacitor C1, a second capacitorC2, a third capacitor C3, and a fourth capacitor C4. The number of theat least one fifth diode D5 is two, including an upper fifth diode D5and a lower fifth diode D5. The number of the at least one sixth diodeD6 is two, including an upper sixth diode D6 and a lower sixth diode D6.

Two ends of the first capacitor C1 are coupled to the secondary sidefirst node Ps1 and the first bridge arm upper node Pu1. Two ends of thesecond capacitor C2 are coupled to the secondary side second node Ps2and the second bridge arm upper node Pu2. Two ends of the thirdcapacitor C3 are coupled to the secondary side first node Ps1 and thefirst bridge arm lower node Pw1. Two ends of the fourth capacitor C4 arecoupled to the secondary side second node Ps2 and the second bridge armlower node Pw2.

A cathode of the upper fifth diode D5 is coupled to the first bridge armupper node Pu1, and an anode of the upper fifth diode D5 is coupled tothe secondary side second node Ps2. A cathode of the upper sixth diodeD6 is coupled to the second bridge arm upper node Pu2, and an anode ofthe upper sixth diode D6 is coupled to the secondary side first nodePs1. An anode of the lower fifth diode D5 is coupled to the first bridgearm lower node Pw1, and a cathode of the lower fifth diode D5 is coupledto the secondary side second node Ps2. An anode of the lower sixth diodeD6 is coupled to the second bridge arm lower node Pw2, and a cathode ofthe lower sixth diode D6 is coupled to the secondary side first nodePs1.

In the circuit structure shown in FIG. 9, when the isolated converteroperates in a first quadrant (not shown), a main current flows throughthe secondary side of the transformer T1, the second capacitor C2, thethird diode D3, the DC positive output node +Vb, the DC negative outputnode −Vb, the second diode D2, the third capacitor C3, and the secondaryside. A first pre-charge current flows through the secondary side, theupper fifth diode D5, the first capacitor C1, and the secondary side. Asecond pre-charge current flows through the secondary side, the fourthcapacitor C4, the lower sixth diode D6, and the secondary side. When theisolated converter operates in a third quadrant (not shown), a maincurrent flows through the secondary side, the first capacitor C1, thefirst diode D1, the DC positive output node +Vb, the DC negative outputnode −Vb, the fourth diode D4, the fourth capacitor C4, and thesecondary side. A first pre-charge current flows through the secondaryside, the upper sixth diode D6, the second capacitor C2, and thesecondary side. A second pre-charge current flows through the secondaryside, the third capacitor C3, the lower fifth diode D5, and thesecondary side.

Please refer to FIG. 10, which shows a single-bus circuit structure ofthe isolated converter with high boost ratio according to a fourthembodiment of the present disclosure. The main difference between FIG.10 and FIG. 9 is that all the diodes D1-D6 in FIG. 9 are replaced bypower switches S1-S6. In this embodiment, the power switches S1-S6 are,for example but not limited to, metal-oxide-semiconductor field-effecttransistors (MOSFETs). By actively controlling the power switches S1-S6,the isolated converter can achieve the function of bidirectionaloperation. Similarly, the diodes in the isolated converters shown inFIG. 5, FIG. 6, and FIG. 9 also can be replaced by power switches sothat the isolated converter can achieve the function of bidirectionaloperation and achieve less conduction loss. In FIG. 5, FIG. 6, and FIG.9, the “diode” is not limited to a diode that is independently packaged,it can be also a parasitic element of the power switch, such as a bodydiode of the MOSFET.

Please refer to FIG. 11, which shows a dual-bus circuit structure of theisolated converter with high boost ratio according to the presentdisclosure. In the same concept of the diode-capacitor circuit, thedual-bus circuit structure is proposed for the specific application,such as a dual-bus output of the UPS's power converter. In comparisonwith the single-bus circuit structure of the isolated converter withhigh boost ratio shown in FIG. 9, the dual-bus circuit structure of theisolated converter with high boost ratio shown in FIG. 11 furtherincludes a center-tapped node Pct. Therefore, balance charging anddischarging operation of the dual-bus output voltage can be implementedthrough the center-tapped structure, and the circuit is similar to FIG.9 and will not be repeated here.

Please refer to FIG. 12, which shows a circuit diagram of a push-pullconverter having the single-bus circuit structure in FIG. 5. Thediode-capacitor circuit shown in FIG. 5 is applied to the push-pullconverter having the single-bus circuit structure to achieve theDC-to-DC converter with high boost ratio and high efficiency. Moreover,it can be controlled by a fixed frequency control manner.

In addition, the diode-capacitor circuit shown in FIG. 6, FIG, 9, andFIG. 10 is also suitable for the push-pull converter (its detailedcircuit diagram is not shown) having the single-bus circuit.

Please refer to FIG. 13, which shows a circuit diagram of a full-bridgeconverter having the single-bus circuit structure in FIG. 5. Thediode-capacitor circuit shown in FIG. 5 is applied to the full-bridgeconverter having the single-bus circuit structure to achieve theDC-to-DC converter with high boost ratio and high efficiency.

In addition, the diode-capacitor circuit shown in FIG. 6, FIG, 9, andFIG. 10 is also suitable for the full-bridge converter (its detailedcircuit diagram is not shown) having the single-bus circuit.

Please refer to FIG. 14, which shows a circuit diagram of a full-bridgeLLC resonant converter having the single-bus circuit structure in FIG.5. The diode-capacitor circuit shown in FIG. 5 is applied to thefull-bridge LLC resonant converter having the single-bus circuitstructure to achieve the DC-to-DC converter with high boost ratio andhigh efficiency.

In addition, the diode-capacitor circuit shown in FIG. 6, FIG, 9, andFIG. 10 is also suitable for the full-bridge LLC resonant converter (itsdetailed circuit diagram is not shown) having the single-bus circuit.

Please refer to FIG. 15, which shows a circuit diagram of a push-pullconverter having the dual-bus circuit structure in FIG. 11. Thediode-capacitor circuit shown in FIG. 11 is applied to the push-pullconverter having the dual-bus circuit structure to achieve the DC-to-DCconverter with high boost ratio and high efficiency.

Please refer to FIG. 16, which shows a circuit diagram of a full-bridgeconverter having the dual-bus circuit structure in FIG. 11. Thediode-capacitor circuit shown in FIG. 11 is applied to the full-bridgeconverter having the dual-bus circuit structure to achieve the DC-to-DCconverter with high boost ratio and high efficiency.

Please refer to FIG. 17, which shows a circuit diagram of a full-bridgeLLC resonant converter having the dual-bus circuit structure in FIG. 11.The diode-capacitor circuit shown in FIG. 11 is applied to thefull-bridge LLC resonant converter having the dual-bus circuit structureto achieve the DC-to-DC converter with high boost ratio and highefficiency.

Although the present disclosure has been described with reference to thepreferred embodiment thereof, it will be understood that the presentdisclosure is not limited to the details thereof. Various substitutionsand modifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the present disclosure as defined in the appended claims.

What is claimed is:
 1. An isolated converter with high boost ratiocomprising: a transformer comprising a secondary side having a secondaryside first node and a secondary side second node, a first bridge armhaving a first diode and a second diode; a cathode of the first diodecoupled to a DC positive output node, an anode of the first diodecoupled to a first bridge arm upper node, an anode of the second diodecoupled to a DC negative output node, and a cathode of the second diodecoupled to a first bridge arm lower node, a second bridge arm having athird diode and a fourth diode; a cathode of the third diode coupled tothe DC positive output node, an anode of the third diode coupled to asecond bridge arm upper node, an anode of the fourth diode coupled tothe DC negative output node, and a cathode of the fourth diode coupledto a second bridge arm lower node, and a boost circuit, comprising: atleast one fifth diode coupled between the first bridge arm and thesecondary side second node, at least one sixth diode coupled between thesecond bridge arm and the secondary side first node, and at least twocapacitors coupled to the secondary side first node and the secondaryside second node.
 2. The isolated converter with high boost ratio inclaim 1, wherein the number of the at least two capacitors is two,including a first capacitor and a second capacitor, wherein two ends ofthe first capacitor are coupled to the secondary side first node and thefirst bridge arm upper node, and two ends of the second capacitor arecoupled to the secondary side second node and the second bridge armupper node, wherein a cathode of the at least one fifth diode is coupledto the first bridge arm upper node, and an anode of the at least onefifth diode is coupled to the secondary side second node; a cathode ofthe at least one sixth diode is coupled to the second bridge arm uppernode, and an anode of the at least one sixth diode is coupled to thesecondary side first node.
 3. The isolated converter with high boostratio in claim 1, wherein the number of the at least two capacitors istwo, including a first capacitor and a second capacitor, wherein twoends of the first capacitor are coupled to the secondary side first nodeand the first bridge arm lower node, and two ends of the secondcapacitor are coupled to the secondary side second node and the secondbridge arm lower node, wherein an anode of the at least one fifth diodeis coupled to the first bridge arm lower node, and a cathode of the atleast one fifth diode is coupled to the secondary side second node; ananode of the at least one sixth diode is coupled to the second bridgearm lower node, and a cathode of the at least one sixth diode is coupledto the secondary side first node.
 4. The isolated converter with highboost ratio in claim 1, wherein the number of the at least twocapacitors is four, including a first capacitor, a second capacitor, athird capacitor, and a fourth capacitor; the number of the at least onefifth diode is two, including an upper fifth diode and a lower fifthdiode; the number of the at least one sixth diode is two, including anupper sixth diode and a lower sixth diode; wherein two ends of the firstcapacitor are coupled to the secondary side first node and the firstbridge arm upper node, two ends of the second capacitor are coupled tothe secondary side second node and the second bridge arm upper node, twoends of the third capacitor are coupled to the secondary side first nodeand the first bridge arm lower node, and two ends of the fourthcapacitor are coupled to the secondary side second node and the secondbridge arm lower node, wherein a cathode of the upper fifth diode iscoupled to the first bridge arm upper node, and an anode of the upperfifth diode is coupled to the secondary side second node; a cathode ofthe upper sixth diode is coupled to the second bridge arm upper node,and an anode of the upper sixth diode is coupled to the secondary sidefirst node, wherein an anode of the lower fifth diode is coupled to thefirst bridge arm lower node, and a cathode of the lower fifth diode iscoupled to the secondary side second node; an anode of the lower sixthdiode is coupled to the second bridge arm lower node, and a cathode ofthe lower sixth diode is coupled to the secondary side first node. 5.The isolated converter with high boost ratio in claim 2, wherein whenthe isolated converter operates in a first quadrant, a main currentflows through the secondary side, the second capacitor, the third diode,the DC positive output node, the DC negative output node, the seconddiode, and the secondary side, a pre-charge current flows through thesecondary side, the at least one fifth diode, the first capacitor, andthe secondary side.
 6. The isolated converter with high boost ratio inclaim 2, wherein when the isolated converter operates in a thirdquadrant, a main current flows through the secondary side, the firstcapacitor, the first diode, the DC positive output node, the DC negativeoutput node, the fourth diode, and the secondary side, a pre-chargecurrent flows through the secondary side, the at least one sixth diode,the second capacitor, and the secondary side.
 7. The isolated converterwith high boost ratio in claim 3, wherein when the isolated converteroperates in a first quadrant, a main current flows through the secondaryside, the third diode, the DC positive output node, the DC negativeoutput node, the second diode, the first capacitor, and the secondaryside, a pre-charge current flows through the secondary side, the secondcapacitor, the at least one sixth diode, and the secondary side.
 8. Theisolated converter with high boost ratio in claim 3, wherein when theisolated converter operates in a third quadrant, a main current flowsthrough the secondary side, the first diode, the DC positive outputnode, the DC negative output node, the fourth diode, the secondcapacitor, and the secondary side, a pre-charge current flows throughthe secondary side, the first capacitor, the at least one fifth diode,and the secondary side.
 9. The isolated converter with high boost ratioin claim 4, wherein when the isolated converter operates in a firstquadrant, a main current flows through the secondary side, the secondcapacitor, the third diode, the DC positive output node, the DC negativeoutput node, the second diode, the third capacitor, and the secondaryside, a first pre-charge current flows through the secondary side, theupper fifth diode, the first capacitor, and the secondary side, a secondpre-charge current flows through the secondary side, the fourthcapacitor, the lower sixth diode, and the secondary side.
 10. Theisolated converter with high boost ratio in claim 4, wherein when theisolated converter operates in a third quadrant, a main current flowsthrough the secondary side, the first capacitor, the first diode, the DCpositive output node, the DC negative output node, the fourth diode, thefourth capacitor, and the secondary side, a first pre-charge currentflows through the secondary side, the upper sixth diode, the secondcapacitor, and the secondary side, a second pre-charge current flowsthrough the secondary side, the third capacitor, the lower fifth diode,and the secondary side.
 11. The isolated converter with high boost ratioin claim 4, wherein the secondary side further comprises a center-tappednode.
 12. An isolated converter with high boost ratio comprising: atransformer comprising a secondary side having a secondary side firstnode and a secondary side second node, a first bridge arm having a firstswitch and a second switch; a first end of the first switch coupled to aDC positive output node, a second end of the first switch coupled to afirst bridge arm upper node, a first end of the second switch coupled toa DC negative output node, and a second end of the second switch coupledto a first bridge arm lower node, a second bridge arm having a thirdswitch and a fourth switch; a first end of the third switch coupled tothe DC positive output node, a second end of the third switch coupled toa second bridge arm upper node, a first end of the fourth switch coupledto the DC negative output node, and a second end of the fourth switchcoupled to a second bridge arm lower node, and a boost circuit,comprising: at least one fifth switch coupled between the first bridgearm and the secondary side second node, at least one sixth switchcoupled between the second bridge arm and the secondary side first node,and at least two capacitors coupled to the secondary side first node andthe secondary side second node.
 13. The isolated converter with highboost ratio in claim 12, wherein the number of the at least twocapacitors is two, including a first capacitor and a second capacitor,wherein two ends of the first capacitor are coupled to the secondaryside first node and the first bridge arm upper node, and two ends of thesecond capacitor are coupled to the secondary side second node and thesecond bridge arm upper node, wherein a first end of the at least onefifth switch is coupled to the first bridge arm upper node, and a secondend of the at least one fifth switch is coupled to the secondary sidesecond node; a first end of the at least one sixth switch is coupled tothe second bridge arm upper node, and a second end of the at least onesixth switch is coupled to the secondary side first node.
 14. Theisolated converter with high boost ratio in claim 12, wherein the numberof the at least two capacitors is two, including a first capacitor and asecond capacitor, wherein two ends of the first capacitor are coupled tothe secondary side first node and the first bridge arm lower node, andtwo ends of the second capacitor are coupled to the secondary sidesecond node and the second bridge arm lower node, wherein a first end ofthe at least one fifth switch is coupled to the first bridge arm lowernode, and a second end of the at least one fifth switch is coupled tothe secondary side second node; a first end of the at least one sixthswitch is coupled to the second bridge arm lower node, and a second endof the at least one sixth switch is coupled to the secondary side firstnode.
 15. The isolated converter with high boost ratio in claim 12,wherein the number of the at least two capacitors is four, including afirst capacitor, a second capacitor, a third capacitor, and a fourthcapacitor; the number of the at least one fifth switch is two, includingan upper fifth switch and a lower fifth switch; the number of the atleast one sixth switch is two, including an upper sixth switch and alower sixth switch; wherein two ends of the first capacitor are coupledto the secondary side first node and the first bridge arm upper node,two ends of the second capacitor are coupled to the secondary sidesecond node and the second bridge arm upper node, two ends of the thirdcapacitor are coupled to the secondary side first node and the firstbridge arm lower node, and two ends of the fourth capacitor are coupledto the secondary side second node and the second bridge arm lower node,wherein a first end of the upper fifth switch is coupled to the firstbridge arm upper node, and a second end of the upper fifth switch iscoupled to the secondary side second node; a first end of the uppersixth switch is coupled to the second bridge arm upper node, and asecond end of the upper sixth switch is coupled to the secondary sidefirst node, wherein a first end of the lower fifth switch is coupled tothe first bridge arm lower node, and a second end of the lower fifthswitch is coupled to the secondary side second node; a first end of thelower sixth switch is coupled to the second bridge arm lower node, and asecond end of the lower sixth switch is coupled to the secondary sidefirst node.
 16. The isolated converter with high boost ratio in claim15, wherein the secondary side further comprises a center-tapped node.